Treatment of H2OsBr6 with 1.5 equiv of C(5)Me(5)H in a mixture of 48% HBr and methanol or ethanol at reflux for 3 h gives a brown precipitate which has the empirical formula "(C(5)Me(5))(2)Os2Br4H"; actually, this product is the unusual salt [(C(5)Me(5))(2)OsH](2)[Os2Br8] where the cation is the conjugate acid of decamethylosmocene and the anion contains an Os-Os triple bond. This is the first [Os(2)X(8)(2-)] Salt that has been prepared directly from a mononuclear precursor. Parent peaks due to both the cation and the anion have been observed by electrospray mass spectrometry and negative-ion fast atom bombardment mass spectrometry, respectively. The presence of a hydride ligand in the cation is shown by the singlet in the H-1 NMR spectrum at delta -15.65; the NMR spectrum is essentially identical with that of [(C(5)Me(5))(2)OsH][PF6]. The brown color of [(C(5)Me(5))(2)OsH](2)[Os2Br8] suggests that the anion adopts an eclipsed conformation (D-4h symmetry) unlike other known salts of [Os2Br82-], and this conclusion has been confirmed by a single-crystal X-ray diffraction experiment. The two crystallographically independent [Os2Br82-] anions lie on sites of 3 and ($) over bar 3 symmetry; the Os-Os vectors are correspondingly disordered along each of three mutually perpendicular directions within the ordered cubes of bromine atoms. The Os-Os distances are 2.219(2) and 2.222(2) Angstrom A in the two independent anions; the average Os-Br distance is 2.46 Angstrom A. The cation adopts a bent metallocene structure with a dihedral angle between the two ring planes of 11 degrees. A byproduct of the reaction of H2OsBr6 With C(5)Me(5)H in ethanol is the osmium(II) carbonyl complex (C(5)Me(5))(2)Os2Br2(mu-CO), where the bridging carbon monoxide ligand (upsilon(co) = 1740 cm(-1)) is evidently generated from decarbonylation of the solvent. Crystal data for [(C(5)Me(5))(2)OsH](2)[Os2Br8] : trigonal, space group P ($) over bar 3, with a = b = 20.732(4) Angstrom A, c = 9.780(2) Angstrom A, gamma = 120 degrees, V = 3640.4(13) Angstrom A(3), Z = 3, R(F) [I > 2 sigma(I)] = 0.0325, and R(wF2) [all data] = 0.0925 for 264 parameters refined without constraints against the 2633 observed data or all 3659 unique data, respectively.